Photos app-like gap between pages in UIScrollView

问题:

UIScrollView in paging mode assumes the pages are located right next to each other, with no gap. However if you open a photo in the Photos app and swipe through photos, you can see that it has some gap between pages. I want these gaps too.

I'm looking for existing solutions if any, or for some more bizarre ideas about implementing the page gaps besides the one I have explained below. Or maybe there's some obvious easy way I am missing?

To be clear: I want the gap to only be visible while scrolling, so I cannot simply inset the page content.

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My plan is to try moving the page content from inside scrollViewDidScroll callback, so that (assuming you're scrolling to the right) initially the target page is slightly offset to the right of its page boundaries, and by the time you arrive at the target page it's back at its proper location, and the source page is slightly offset to the left of its boundaries. (Or maybe instead of moving things continuously, I'll be better off shifting the offsets, say, exactly halfway between pages.)

I'm the author of the ScrollingMadness article+example that I've been referring some people to here. I've implemented progammatic zooming, and got in-photo zooming+scrolling working together with inter-photo paging. So I know how to play with UIScrollView, and am looking for the advanced stuff.

Please don't point me at TTScrollView. I've already pointed many people to it myself, but I consider it's feel too far from the native UIScrollView behaviour, and do not want to use it in my projects.

回答1:

Note that this answer is quite old. The basic concept still works but you should not be hard coding view sizes in iOS7 and 8. Even if you ignore that advice, you should not use 480 or 330.

Have you tried making the frame of the UIScrollView slightly larger than the screen (assuming that you want to display your images fullscreen and then arranging your subviews on the same slightly-larger-than-the-screen boundaries.

#define kViewFrameWidth 330; // i.e. more than 320

CGRect scrollFrame;
scrollFrame.origin.x = 0;
scrollFrame.origin.y = 0; 
scrollFrame.size.width = kViewFrameWidth;
scrollFrame.size.height = 480;

UIScrollView* myScrollView = [[UIScrollView alloc] initWithFrame:scrollFrame];
myScrollView.bounces = YES;
myScrollView.pagingEnabled = YES;
myScrollView.backgroundColor = [UIColor redColor];

UIImage* leftImage = [UIImage imageNamed:@"ScrollTestImageL.png"];
UIImageView* leftView = [[UIImageView alloc] initWithImage:leftImage];
leftView.backgroundColor = [UIColor whiteColor];
leftView.frame = CGRectMake(0,0,320,480);

UIImage* rightImage = [UIImage imageNamed:@"ScrollTestImageR.png"];
UIImageView* rightView = [[UIImageView alloc] initWithImage:rightImage];
rightView.backgroundColor = [UIColor blackColor];
rightView.frame = CGRectMake(kViewFrameWidth * 2,0,320,480);

UIImage* centerImage = [UIImage imageNamed:@"ScrollTestImageC.png"];
UIImageView* centerView = [[UIImageView alloc] initWithImage:centerImage];
centerView.backgroundColor = [UIColor grayColor];
centerView.frame = CGRectMake(kViewFrameWidth,0,320,480);

[myScrollView addSubview:leftView];
[myScrollView addSubview:rightView];
[myScrollView addSubview:centerView];

[myScrollView setContentSize:CGSizeMake(kViewFrameWidth * 3, 480)];
[myScrollView setContentOffset:CGPointMake(kViewFrameWidth, 0)];

[leftView release];
[rightView release];
[centerView release];

Apologies if this doesn't compile, I tested it in a landscape app and hand edited it back to portrait. I'm sure you get the idea though. It relies on the superview clipping which for a full screen view will always be the case.

回答2:

So I don't have enough "rep" to post a comment on the answer above. That answer is correct, but there is a BIG issue to be aware of:

If you're using a UIScrollView in a viewController that's part of a UINavigationController, the navigation controller WILL resize the frame of your scrollView.

That is, you have an app that uses a UINavigationController to switch between different views. You push a viewController that has a scrollView and you create this scrollView in the viewController's -init method. You assign it a frame of (0, 0, 340, 480).

Now, go to your viewController's -viewDidAppear method, get the frame of the scrollView you created. You'll find that the width has been reduced to 320 pixels. As such, paging won't work correctly. You'll expect the scrollView to move 340 pixels but it will, instead, move 320.

UINavigationController is a bit notorious for messing with subviews. It moves them and resizes them to accommodate the navigation bar. In short, it's not a team player -- especially in this case. Other places on the web suggest that you not use UINavigationController if you need precise control over your views' size and locations. They suggest that, instead, you create your own navigationController class based on UINavigationBar.

Well that's a ton of work. Fortunately, there's an easier solution: set the frame of the scrollView in your viewController's -viewDidAppear method. At this point, UINavigationController is done messing with the frame, so you can reset it to what it should be and the scrollView will behave properly.

This is relevant for OS 3.0. I have not tested 3.1 or 2.2.1. I've also filed a bug report with Apple suggesting that they modify UINavigationController with a BOOL such as "-shouldAutoarrangeSubviews" so that we can make that class keep its grubby hands off subviews.

Until that comes along, the fix above will give you gaps in a paginated UIScrollView within a UINavigationController.

回答3:

Apple has released the 2010 WWDC session videos to all members of the iphone developer program. One of the topics discussed is how they created the photos app!!! They build a very similar app step by step and have made all the code available for free.

It does not use private api either. Here is a link to the sample code download. You will probably need to login to gain access.

http://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/wa/getSoftware?code=y&source=x&bundleID=20645

And, here is a link to the iTunes WWDC page:

http://insideapple.apple.com/redir/cbx-cgi.do?v=2&la=en&lc=&a=kGSol9sgPHP%2BtlWtLp%2BEP%2FnxnZarjWJglPBZRHd3oDbACudP51JNGS8KlsFgxZto9X%2BTsnqSbeUSWX0doe%2Fzv%2FN5XV55%2FomsyfRgFBysOnIVggO%2Fn2p%2BiweDK%2F%2FmsIXj

回答4:

The way to do this is like you said, a combination of a few things.

If you want a gap of 20px between your images, you need to:

First, expand your scroll view's total width by 20px and move it left by 10px.

Second, when you lay out the xLoc of your images, add 20px for each image so they're spaced 20px apart. Third, set the initial xLoc of your images to 10px instead of 0px.

Fourth, make sure you set the content size of your scroll view to add 20px for each image. So if you have kNumImages images and each is kScrollObjWidth, then you go like this:

[scrollView setContentSize:CGSizeMake((kNumImages * (kScrollObjWidth+20)),  kScrollObjHeight)];

It should work after that!

回答5:

This is just a hunch, so apologies if completely wrong, but is it possible that the contentSize is just set to slightly wider than the screen width.

The correct information is then rendered within the view to the screen width and UIScrollView takes care of the rest ?

回答6:

Maybe you want to try UIScrollView's contentInset property?

myScrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 10.0);

回答7:

I just thought I'd add here for posterity the solution I ended up going with. For a long time I've been using Bryan's solution of adjusting the frame in -viewDidAppear, and this has worked brilliantly. However since iOS introduced multitasking I've been running into a problem where the scroll view frame gets changed when the app resumes from the background. In this case, -viewDidAppear was not being called and I couldn't find a delegate method that would be called at the right time to reverse the change. So I decided to make my scroll view a subview of my View Controller's view, and this seemed to fix the problem. This also has the advantage of not needing to use -viewDidAppear to change the frame - you can do it right after you create the scroll view. My question here has the details, but I'll post them here as well:

CGRect frame = CGRectMake(0, 0, 320, 460);
scrollView = [[UIScrollView alloc] initWithFrame:frame];

// I do some things with frame here

CGRect f = scrollView.frame;
f.size.width += PADDING; // PADDING is defined as 20 elsewhere
scrollView.frame = f;

[self.view addSubview:scrollView];

回答8:

To avoid messing with UIScrollView's frame, you could subclass UIScrollView and override layoutSubviews to apply an offset to each page.

The idea is based on the following observations:

1. When zoomScale !=1, the offset is zero when it is at the left / right edge
2. When zoomScale ==1, the offset is zero when it is at the visible rect centre

Then the following code is derived:

- (void) layoutSubviews
{
    [super layoutSubviews];

    // Find a reference point to calculate the offset:
    CGRect bounds = self.bounds;
    CGFloat pageGap = 8.f;
    CGSize pageSize = bounds.size;
    CGFloat pageWidth = pageSize.width;
    CGFloat halfPageWidth = pageWidth / 2.f;
    CGFloat scale = self.zoomScale;
    CGRect visibleRect = CGRectMake(bounds.origin.x / scale, bounds.origin.y / scale, bounds.size.width / scale, bounds.size.height / scale);
    CGFloat totalWidth = [self contentSize].width / scale;
    CGFloat scrollWidth = totalWidth - visibleRect.size.width;
    CGFloat scrollX = CGRectGetMidX(visibleRect) - visibleRect.size.width / 2.f;
    CGFloat scrollPercentage = scrollX / scrollWidth;
    CGFloat referencePoint = (totalWidth - pageWidth) * scrollPercentage + halfPageWidth;

    // (use your own way to get all visible pages, each page is assumed to be inside a common container)
    NSArray * visiblePages = [self visiblePages];

    // Layout each visible page:
    for (UIView * view in visiblePages)
    {
        NSInteger pageIndex = [self pageIndexForView:view]; // (use your own way to get the page index)

        // make a gap between pages
        CGFloat actualPageCenter = pageWidth * pageIndex + halfPageWidth;
        CGFloat distanceFromRefPoint = actualPageCenter - referencePoint;
        CGFloat numOfPageFromRefPoint = distanceFromRefPoint / pageWidth;
        CGFloat offset = numOfPageFromRefPoint * pageGap;
        CGFloat pageLeft = actualPageCenter - halfPageWidth + offset;

        view.frame = CGRectMake(pageLeft, 0.f, pageSize.width, pageSize.height);
    }
}